Flat panel displays are used as displays for computers, etc. There are various types of flat panel displays, and LCDs that use liquid crystals are widely used. A simple matrix liquid crystal display panel is representative of these.
FIG. 11 shows a sketch of a simple matrix LCD panel. This simple matrix LCD panel 10, as shown in FIG. 11, is constructed from liquid crystals sandwiched by scanning electrodes X1, X2, . . . , XN and signal electrodes Y1, Y2, . . . , YM, and each point of intersection of scanning electrodes X and signal electrodes Y constitutes a pixel.
FIG. 12 shows a block diagram of a simple matrix LCD panel device. This simple matrix LCD panel 20 is composed of a simple matrix LCD panel 10, scanning electrode drivers C1, . . . , Cn, signal electrode drivers S1, S2, . . . , Sm, a controller 12 that controls scanning electrode drivers C and signal electrode drivers S, and a power supply 16.
LCD panel 10 transmits display signals to each pixel by scan-driving (time allocation driving) and constitutes a screen. That is, one line is displayed by sending the relevant display signal from signal electrodes Y to the column selected by scanning electrode X. The selected signals are scanned sequentially from the top and one frame (screen) is scanned per cycle.
FIG. 13 shows an example of the voltage waveforms applied to scanning electrodes X and signal electrodes Y of LCD panel 10 in a 6-level drive method. In FIG. 13, (a) is the voltage waveform applied to scanning electrodes X, (b) and (c) are voltage waveforms applied to signal electrodes Y, and (d) is a voltage waveform (absolute value) applied to each pixel.
Here, data output from controller 12 to scanning electrode drivers C and signal electrode drivers S are signals with a logical amplitude of 0-5 V. 0 V, 2.5 V, 5 V, 27.5 V and 30 V are fed to scanning electrode drivers C, and 0 V, 5 V, 25 V and 30 V are fed to signal electrode drivers S.
The 6-level drive method of a simple matrix LCD panel using the voltage waveforms in FIG. 13 will be explained below. Note that to simplify the explanation the display on LCD panel 10 will have two values: on/off (white/black).
When liquid crystal material is driven by direct current, ions accumulate on one side, thus the liquid crystal material is quickly degraded, so it must be driven by alternating current. Thus, as shown in FIG. 13(a), for scanning electrodes X there are two nonselection voltages of 2.5 V and 27.5 V and two selection voltages of 30 V and 0 V. Also, as shown in FIGS. 13(b) and (c) , for signal electrodes Y there are two nonselection (pixel off) voltages of 5 V and 25 V and two selection (pixel on) voltages of 0 V and 30 V. The turning on/off of each pixel is controlled by combining each of the voltages stated above. The selection voltage of a signal electrode Y is 0 V when the selection voltage of a scanning electrode X is 30 V, and the selection voltage of a signal electrode Y is 30 V when the selection voltage of a scanning electrode X is 0 V. Therefore, application of 30 V at the pixel located at the point of intersection of the scanning electrode X and signal electrode Y will turn on the given pixel. On the other hand, the nonselection voltage of a signal electrode Y is 5 V when the selection voltage of a scanning electrode X is 30 V, and the nonselection voltage of a signal electrode Y is 25 V when the selection voltage of a scanning electrode X is 0 V. Therefore, a voltage of 25 V is applied to the corresponding pixel, and the given pixel will be off. Also, when the 2.5 V nonselection voltage is applied to each scanning electrode X, 0 V or 5 V will be applied to each signal electrode Y, and when the 27.5 V nonselection voltage is applied to each scanning electrode X, 25 V or 30 V will be applied to each signal electrode Y. Therefore, a voltage of 2.5 V will be applied to each pixel of each scanning electrode X that is not selected, and each of the given pixels will remain off. As shown in FIG. 13, each voltage of 0 V, 2.5 V, 27.5 V and 30 V must be applied to scanning electrodes X, and each voltage of 0 V, 5 V, 25 V and 30 V must be applied to signal electrodes Y, so that scanning electrode drivers C and signal electrode drivers S require a drive circuit whose output voltage range is 0-30 V, that is, a high breakdown voltage transistor. Generally, control data from controller 12 that controls scanning electrode drivers C and signal electrode drivers S, are 5 V system (0-5 V signal amplitude) signals. Therefore, a level shift circuit must be installed inside scanning electrode drivers C and signal electrode drivers S and the 5 V system signals converted to logical amplitude signals of 0-30 V.
A high breakdown voltage transistor that outputs a voltage of up to 30 V requires a relatively large area on an IC chip, so that the area of the driver IC chip that composes scanning electrode drivers C and signal electrode drivers S will become greater, and this is one cause of the increased cost of scanning electrode drivers C and signal electrode drivers S. Also, the scanning electrode drivers C and signal electrode drivers S require a level shift circuit, so that the increase in IC chip area caused by this level shift circuit will increase the cost of scanning electrode drivers C and signal electrode drivers S.
A typical simple matrix LCD panel 10 has a 640.times.480 dot construction, so that 480 scanning electrodes X are needed in a black-and-white LCD panel, and 640 signal electrodes Y are needed. On the other hand, in color LCD panels, three R, G, and B signal electrodes Y are needed for one pixel, so that 480 scanning electrodes X are needed, and 1920 signal electrodes Y are needed. When the LCD panel becomes large and highly precise in this way, the number of signal electrodes Y will be very large, compared to the number of scanning electrodes X. Therefore, as the number of signal electrodes Y increases, the number of signal electrode drivers S of simple matrix LCD device 20 will increase, and the cost of display device 20 will increase because of the increased number of signal electrode drivers S.
Therefore, an object of the present invention is to provide a drive circuit where the area the circuit forms on an IC chip is small.
Also, an object of the present invention is to provide a drive circuit suitable for liquid crystal panel display devices.